Cosmic Rays Come from Exploding Stars: An Interview with Stefan Funk

SCIENTISTS HAVE KNOWN ABOUT COSMIC RAYS FOR A CENTURY. But these high-energy subatomic particles, which stream through space at nearly the speed of light and crash into the Earth’s upper atmosphere, have been mostly a mystery. The primary reason: researchers have been unable to tell where they come from, or how they’re born.

Now, a research team led by the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC) at Stanford University and SLAC has discovered evidence that cosmic rays are born in the shock waves of distant supernovae.

Using data from NASA’s Fermi Gamma-ray Space Telescope, the research team led by Dr. Stefan Funk of KIPAC was able to track gamma rays – the most energetic form of electromagnetic radiation, or light – back to the remnants of supernovae explosions where they were born as decay products of collisions between cosmic rays and lower energy particles. The finding offers the first astrophysical evidence for how cosmic rays are produced, as well as where they are generated: in the shock waves that emanate from an exploded star.

Dr. Funk and his team confirmed a chain of events that researchers have theorized for years: The shock waves from supernovae accelerate subatomic particles called protons to near light speeds, turning them into cosmic rays. These cosmic rays collide with low-energy protons residing in gas and dust gathered by the shock waves, and the collisions give rise to other subatomic particles called pions. Pions rapidly decay into gamma rays with a very specific distribution of energies. Gamma rays below a certain energy related to the rest mass of the pion are completely absent from the signal. The detection of gamma rays with that characteristic distribution of energies, in the direction of two known supernovae remnants, provided evidence for the entire process – and a smoking gun for how cosmic rays are produced.

While other objects in the universe generate cosmic rays, most probably active galactic nuclei located far beyond our own Milky Way galaxy, supernovae in our own galactic neighborhood are thought to produce a large fraction of the cosmic rays that impact Earth.

The study by Dr. Funk and his team of researchers appears in the Feb. 15 issue of Science. Dr. Funk spoke this week with The Kavli Foundation about the discovery, what it means, and what the next steps are in his team’s work.

The following is an edited transcript of his remarks.

THE KAVLI FOUNDATION: Dr. Funk, can you begin by telling us – what are cosmic rays?

STEFAN FUNK: Cosmic rays are made up of high-energy particles that reach the upper atmosphere from outer space. They consist mostly of protons, in other words hydrogen nuclei, but they also can consist of nuclei of Helium or heavier elements, of electrons and other subatomic particles. They were discovered in 1912 by an Austrian named Victor Hess. He demonstrated in a series of balloon flights that the radiation increased at higher altitudes, and this was observational proof that the source of radiation was coming from outer space.

Cosmic rays arrive here going at near the speed of light. Their energies can be enormous - much higher than what we can produce in our terrestrial accelerators, such as the Large Hadron Collider (LHC) in Geneva.

When stars explode, the supernovas send off shock waves like the one shown in this artist's rendition, which accelerate protons to cosmic-ray energies through a process known as Fermi acceleration. (Credit: Greg Stewart, SLAC National Accelerator Laboratory)

TKF: For anyone unfamiliar with cosmic rays, are they dangerous?

FUNK: They could be, but the good thing about the Earth is that it has a shield. When these high-energy protons – i.e. cosmic rays – enter the atmosphere, they interact with particles in the atmosphere and as a consequence lose most of their energy. So, very few of the cosmic rays that arrive in the upper atmosphere actually make it to the ground. Only the very highest energy protons, the so-called ultra-high energy cosmic rays, reach the ground. Therefore here on the ground, most of the radiation we get is from the Earth itself and cosmic rays account for only a few percent of the total. This changes as you go higher in the atmosphere. During trans-Atlantic flights, for example, you’re much more exposed to cosmic rays. If you leave the atmosphere, you're totally exposed to all the cosmic rays, and this is a significant hazard for long-duration spaceflight.

TKF: And why is it so hard to tell where cosmic rays originate?

FUNK: In the past 100 years we’ve studied them in great detail as they arrive here on Earth, but it’s been much more difficult to pinpoint where they come from. That’s because protons are deflected by magnetic fields that permeate our galaxy – in other words, they’re deflected from their source on their way to us so they don’t point back in that direction. And that means we cannot do astrophysics with these protons because it’s so hard to find where they come from.

In order to understand the origin and acceleration of cosmic ray protons, researchers used data from the Fermi Gamma-ray Space Telescope and targeted W44 and IC 443, two supernova remnants thousands of light years away. Both turned out to be strong sources of gamma rays, but not at energies below what neutral pion decay would produce - the observational proof scientists had been looking for. (Credit: NASA/DOE/Fermi LAT Collaboration).

FUNK: In our study we turned to “neutral messengers” to tell us where at least some cosmic rays originate. We studied gamma rays, the highest energy photons in the electromagnetic spectrum. Because they are photons, they’re electrically neutral and so they’re not deflected by magnetic fields in the galaxy; they travel in a straight line. Theory predicts that the gamma rays coming directly from supernovae remnants will have a particular energy distribution if they’re generated by the decay of pions, which are only produced by the collision of high-energy protons – cosmic rays – with lower energy protons. So finding gamma rays coming directly from supernovae remnants, with that signature energy distribution – the complete absence of gamma rays with energies below the pion restmass, was our smoking gun. In this case, the cosmic rays interact with interstellar material in the immediate vicinity of the supernova shock wave and hence don’t make it as cosmic rays here to Earth, but the gamma rays produced in these proton-proton collisions do, and those gamma rays are what we measured.

TKF: Was finding the gamma rays difficult?

FUNK: To observe these gamma rays in space, you need a special telescope and you need a detector on the telescope that is sensitive to gamma rays with the energy predicted for this process. The Fermi Gamma-ray Space Telescope was sensitive enough to detect the kind of gamma-ray signature we were looking for. We have detected several additional supernovae remnants but detecting this smoking gun feature so far was only possible in the two brightest ones – at those low energies the analysis is extremely complex and it is very much an effort from the whole LAT team that allowed us to reach those low energies. So it was the combination of having a sensitive-enough instrument and very bright supernova remnants that allowed us to study this.

TKF: Scientists have long believed that supernovae remnants are one major source of cosmic rays, correct?

FUNK: Yes. The idea that the remnants of exploded stars should be the dominant source goes back to the 1960s. There are very few astrophysical objects in our galaxy that could sustain the constant stream of cosmic rays that reaches Earth. Back in the 1960s, researchers calculated that if supernova remnants are somehow able to put 10 percent of their explosion energy into the acceleration of protons, they can sustain the amount of cosmic rays that we see impacting Earth. In our study, we’ve shown that the remnants of the two supernova explosions we targeted in fact convert about the right level of the explosion energy into the production of cosmic rays. But of course, we haven’t confirmed that for all supernovae remnants in the Milky Way – although all of them, collectively, are thought to contribute to the cosmic rays that impact Earth.

TKF: What is the estimated number of supernova remnants in the Milky Way Galaxy?

FUNK: Supernovae in our galaxy occur at a rate of two to three per century. We think that the shock waves from those explosions last about 50,000 years, and they accelerate protons continuously during that time. That means at any given moment you expect to have about a thousand objects actively accelerating cosmic rays.

Finding evidence for the acceleration of protons has long been a key issue in the efforts to explain the origin of cosmic rays. This pair of spectra from two supernova remnants, shown here with data from various satellites and wavelengths, are the "smoking gun" that researchers have been looking for. The Fermi Large Area Telescope's observations fit neatly with predictions of neutral pion decay. (Credit: NASA/DOE/Fermi LAT Collaboration, Chandra X-ray Observatory, ESA Herschel/XMM-Newton)

TKF: The press release states, "A new study confirms what scientists have long suspected." Was there anything surprising about your findings?

FUNK: Making this discovery was one of the goals of the Fermi Large Area Telescope, but it’s surprising that the instrument has worked so well. Our discovery was made after the telescope had been operating for four years, and our instrument team worked hard to improve the instrument’s sensitivity. It was very much a team effort that allowed us to make this measurement.

TKF: The next step in your research is to study in more detail how the shock waves from supernovae can accelerate protons to near the speed of light. Why is this your next step, and how will you do this?

FUNK: We are interested in learning more about how protons are accelerated by the shock waves of a supernova over time. So, the shock waves plow through the interstellar medium and sweep up material for about 50,000 years. The process of accelerating protons is a very slow one, and in order to gain very high energies, we believe that these particles have to cross the shock front many times. This process is actually called “Fermi acceleration.”

These shock waves get slower and slower over time as they sweep up material, and we would like to understand in which of their evolutionary stages they are best able to accelerate particles. In which of their evolutionary stages over this lifetime of 50,000 years do they accelerate a particle to which energy? To understand this, we need to study a large number of supernova remnants. We can then separate them into evolutionary stages, see some commonalities between stages, and then try to learn something more about how the particles are accelerated.

By studying these stages, we can learn more about how much energy is needed to generate cosmic rays, and how long after an initial supernova they are generated. These insights will help us better understand the ocean of high-energy particles that permeates space and showers the Earth with radiation.